Jour. N. Y. Bot. Garden 26(308): 178-187 (Aug. 1925)
SEEDS: THEIR TRICKS AND TRAITS1
WILLIAM CROCKER

1 Abstract of an illustrated lecture given in the Museum Building of The New York Botanical Garden on Saturday afternoon, July if, 1925, by the Director of the Boyce Thompson Institute far Plant Research.

The fairy tale of the germination of three thousand year old wheat from the tombs of the Egyptian Pharaohs was exploded some years ago. Authentic records show that wheat under usual conditions of storage does not retain its vitality over twenty years. Many records on the other hand show very much greater longevity than this for various other seeds. Not the least interesting of these records is one just published by Ohga, a Japanese botanist. He has excavated from the Pulantien Plain of South Manchuria viable seeds of Nelumbo nucifera, East Indian Lotus, that he believes have lain in the moist peat at least 120 years and more likely between 200 and 400 years.

It is interesting to see what has happened to the Pulantien Plain since the Nelumbo plants grew there and deposited their seeds. The seeds are found near the top of a deposit of peat that is now 13 to 20 inches thick. The plants must have grown there when the plain was a shallow receding lake. Since then 20 to 30 inches of fine silt soil have been deposited over the peat, undoubtedly blown by the wind from the adjacent Gobi desert. The Pulantien River has cut such a deep channel through the old lake bed that the bed of this river is now about forty feet below the peat deposit that contains the seeds. Trees that do not ordinarily grow in water are now growing on the silt deposit over the peat where the lake once existed. Some of these are 120 to 130 years old. Mr. U. Liu, the director of the Liantung Bank in Darien, states that his ancestors moved into and cultivated this basin about 200 years ago. Tombs in the cemetery at the village of Liu-Chiatung confirm this statement. All of this evidence leaves little doubt that Ohga is right in placing the age of these seeds as greater than 120 years and more likely 200 to 400 years.

But why have these seeds not germinated before this? Some one may suggest that they did not get sufficient oxygen, buried as they were in the peat two or three feet below the surface of the soil. No, there is another and better reason! Besides it is probable that these seeds can germinate in absence of external oxygen as can the water plantain and many other seeds of water plants. The real answer is that these seeds have not yet lain in the moist earth long enough to absorb water! If the coats are filed (or eaten away sufficiently with concentrated sulfuric acid and then washed) they swell. Every one of these seeds treated in this way to date has germinated and Ohga has some very fine plants as a result. The seeds are not only alive but they are all alive and vigorous.

The seeds of Nelumbo are technically termed "hard seeds," that is, an outer layer of the coats prevents the absorption of water so that the seeds may lie in water or moist soil for years with only now and then one swelling and with many remaining hard for years, and perhaps centuries.

Hard seeds are common amongst clovers, alfalfas, beans, peas, indigo plants, locusts, and many other members of the bean family as well as in the mallow family, the water-lily family and several other families of plants. This character causes much annoyance and considerable economic loss in the first-named family of very important food and forage plants. If one were to gather ripe seeds of red clover and hull them in the palm of his hand, he would find 85 to 90 per cent. of them incapable of absorbing water even after weeks of soaking. Fortunately when the red clover is passed through the huller the coats are cracked, as indicated by the fact that a larger percentage of them swell although the cracks in the main are not visible either to the naked eye or under the microscope.

I gathered sweet clover seeds last fall and threshed them by hand. Up to March 1st only 2 per rent had swollen after lying in water for months. Because of the greater hardness and less adequate treatment in the threshing process, sweet clover causes greater economic annoyance than red clover.

There is much evidence that a crop of "hard seeds" shed into the soil will lie there for years with now and then a few swelling and germinating. One crop of seeds may thus seed the soil down for fifty years or perhaps in some cases for one or more centuries.

Are hard seeds the only ones that lie in the soil for long periods dormant and capable of germination? By no means! Many seeds that absorb water readily do the same thing.

The cocklebur is an example. If one splits a bur lengthwise through the two horns he will see that it bears two seeds—one a little higher in the bur than the other. The lower seed in the bur germinates the first year after ripening while the upper frequently germinates the second year or later. The delay in the upper seed is due to a very thin membrane that surrounds the embryo and reduces the oxygen supply to it below that needed for germination. The upper seed germinates only when the coat is broken by freezing—becomes ineffective through decay or when the temperature is high enough (91° F.) to overcome the effect of the coat. When the upper seed germinates, depends upon when it meets one of these contingencies. Some other seeds that absorb water readily, lie dormant in the soil because enclosing membranes reduce the oxygen supply to the embryos.

Seeds of the water plantain lie dormant in water for years because the seed contents do not swell with enough force to break the rather delicate but strong coats. These seeds cannot germinate, much as a chicken cannot hatch if it is unable to break the shell. The chicken soon dies under such circumstances but the embryo of the water plantain rests easy for years, if necessary, until some change, slow or sudden, internal or external, brings about the necessary breaking of the coats. Some other seeds remain dormant, due to the same mechanism as the water plantain.

Dormancy in rose seeds is a very interesting story but time and space will not permit its telling in detail. Rose hybridizers stratify these seeds in a cold place. Under this condition the seeds gradually germinate through a period of five to seven years—a long time to wait for the last seedling of the hybrids. Fortunately, science has stepped in here and helped. First, it has shown that the embryos themselves are dormant and that these embryos must go through some very definite and important chemical changes, called after-ripening, before they are ready to grow. Second, science has shown that these changes occur most rapidly when the germinator or stratification bed is held at 41° F. Under this condition the seeds of all the rose species tested to date will after-ripen and germinate within 140 days. Some of the hybrids offer much greater difficulty. It has also been shown that when the stratification bed rises considerably in temperature the seeds go back into the dormant condition. Thus in the old stratification practice much that is gained in the winter is lost the following summer. This old method was a good one but it lacked one very essential feature—that of controlled temperature in the stratification bed. The new method saves time.

Many seeds are like the rose in having dormant embryos and in requiring a low temperature period in a germinator for after-ripening. This is true of basswood, juniper, fall-seeding maples, haws, peaches, apples and many other seeds. The nature of the soil is sometimes very important. Cotoneaster seeds after-ripen readily in acid peat at 41 F., but not in sand at the same temperature.

The last few paragraphs give some of the mechanisms by which seeds are able to distribute their germination over a long space of time so that some will always be ready to germinate and seed down the earth. Of what advantage is this to wild plants? Did you ever get your garden so well hoed that no more weeds came op in it? It is possible that that could not be done in one generation even if no weeds were allowed to ripen in the garden and no weed seeds were carried in from outside. Many seeds show time distribution in their germination. This gives weeds and other wild plants great persistence. A great number of seeds per plant and time distribution in the germinations are cardinal features in the survival of species.

The farmers of the South Downs of England find that when they plow meadows that have been growing for twenty, thirty, or forty years, black mustard comes up in profusion, although this weed was not present in the meadows. These farmers believe that the seeds lie in the soil dormant but viable during the entire life of the meadow. Numerous other observations of an entirely new type of vegetation appearing upon soil freshly turned up by the digging of walls, ditches, the removal of buildings, and the plowing of old meadows and pastures have been cited as proving that seeds may lie in the soil quiescent and viable for 25, or even 50 years, and germinate when the soil is loosened up and the seeds given proper air, water, and perhaps light conditions, for germination.

Fortunately these claims, like all claims that are not based on absolutely conclusive evidence, have their "Doubting Thomases." The Thomases say that birds carried the black mustard seeds to the meadows or that s few of the black mustard plants grew on the meadows unobserved and kept them seeded. They even suggest that squirrels buried the Nelumbo seeds that Ohga digs from the Polantien basin.

The great abundance of the seeds is against the contentions of the doubters in each case, also, the fact that no Nelumbo plants are now growing near the Pulantien basin is against their claims. If neither of these were true the doubters must still acknowledge that it would take the birds and squirrels several years to do all this seeding, so the seeds must lie dormant in the soil for several years at least.

The fatal evidence against the claims of the doubters comes from another direction.

In 1879, or forty-six years ago, Doctor Beal, of Michigan Agricultural College, buried twenty lots of seeds, each including 50 seeds of twenty different species of plants, mainly weeds. These were placed in moist sand in pint bottles. The uncorked bottles were buried on the campus, twenty inches below the surface of the soil, with the nozzle of the bottle pointing downward to prevent filling with water while the sand in the bottle would remain moist. Each five years one bottle has been taken up and the seeds tested for germination. The 40th year test gave the following results:

  Percentage germinated
after being buried 40 years.
Amaranthus retroflexus—pigweed   2 Per Cent  
Ambrosia elatior—ragweed   4  
Brassica nigra—black mustard   18  
Bromus secalinus—brome grass   0  
Bursa Bursa-pastoris—shepherd's purse   0  
Erechtites hieracifolia   0  
Euphorbia maculata   0  
Lepidium virginicum—pepper grass   2  
Lychnis Githago   0  
Anthemis Cotula—dog fennel   0  
Malva rotundifolia—dwarf mallow   0  
Oenothera biennis—evening primrose   38  
Plantago major—plantain   10  
Polygonum Hydropiper—smartweed   0  
Portulaca oleracea—purslane   2  
Rumex crispus—yellow dock   18  
Chaetochloa lutescens   0  
Alsine media   0  
Trifolium repens—white clover   0  
Amaranthus graecizans—tumble weed   66  
(Chenopodium album)—lamb's quarter   *2  

*Germinated though not recorded as buried.

From this table it is seen that ten of the twenty-two species of seeds that were buried still retain their vitality after forty years in the moist soil. Two others, Verbascum Thapsus  and Bursa Bursa-pastoris, showed viable seeds at the 35-year test. On the basis of all the seeds buried, 8.2 per cent. of them are still alive but have failed to germinate after forty years in the moist soil. The remaining 12 samples will be taken up at ten-year instead of five-year periods, thus giving 120 years more for the experiment to run.

Over 20 years ago the United States Department of Agriculture started even much more extensive experiments with buried seeds. They buried seeds of 105 different species in great numbers and in a variety of conditions. Their 20-year tests confirm Doctor Beal's conclusions so far as seeds of wild plants are concerned. Seeds of various grains and some other cultivated plants did not remain alive even five years. Evidently they were not dormant enough to resist germination so they germinated and the deeply buried seedlings perished. Yet the seeds of many cultivated plants are still persisting after being buried 20 years. This is true of timothy, Kentucky blue grass, celery, tobacco, and red clover. It is probable that many more were capable of germination than either Beal's results or those of the Department of Agriculture shows, for neither treated the seeds to overcome dormancy, a thing Ohga found necessary.

Doctor Ewart, an authority on the longevity of seeds, states that seeds will remain alive longer in the soil than in dry storage. This he means to apply only to "hard seeds." The same is true, however, of some seeds that absorb water readily.

A number of years ago I tested more than a dozen different collections of dry-stored twenty-three-year-old pigweed seeds (Amaranthus retroflexus) and not one was alive. Yet according to Doctor Beal's experiments these seeds are still alive after forty years in the moist soil. It is still an unexplained mystery how imbibed seeds can lie in the soil so long without entirely exhausting their stored foods by respiration

Why do dormant seeds germinate so abundantly when an old meadow is plowed or the soil otherwise opened up? Are the seeds continually germinating at this rate and the seedlings being killed by competing vegetation and deep burial or does stirring the soil arouse the dormant seeds? If one takes the seeds of pigweed that have been in the germinator dormant for weeks and gives them a thorough rubbing in the palm of the hand he will find a large percentage of them germinating soon afterwards. Many other rases can be cited of slight agitation arousing resting seeds. There is no doubt that cultivation or loosening up the soil has a similar effect in arousing some seeds from dormancy.

It would be interesting to consider in detail the annoyance and economic loss caused by the rest period in seeds and to see what practical men and scientists are doing to overcome these. Space, however, will permit of only two or three examples. We have already referred to rose seeds and to clovers and various other legumes in this connection. It might also be seated that rather generally when a man wants to cultivate a wild form, whether for decorative or other purposes, he meets an annoying delay in the germination of the seeds. This was true when Germany introduced wild legumes for forage in the eighties and it is true today when nurserymen and florists try to introduce some of the wonderful wild forms for decorative purposes.

Up to the last two or three decades it has been assumed that the seeds of the cereal graina have no rest period. They may have largely lost such a rest period by a process of selection, for if any are delayed long in germination they will not become a part of the harvest. The six months rest period in wild oats in contrast to the very transient rest period in cultivated oats is perhaps evidence for this. We have lately discovered, however, that the rest period in the cereals is sometimes of considerable economic significance.

To the more northern of the winter wheat states the time between threshing and sowing the next year's crop is only a few weeks. The laws require that the wheat must be tested for vitality and purity and officially declared to by the State Seed Analyst before it can be put on the market for seeds. This means that the test must be made very soon after threshing. Now it happens that wheat has a rest period of a few weeks and that this dormancy is lost in dry storage. The State Seed Analyst declared the seed poor on the basis of his tests made immediately after threshing and yet the seed grew perfectly a few weeks later when it was sown. This caused much trouble between the state authorities and the traders in seed wheat. This led to a thorough-going research and fortunately very simple methods were found for germinating the dormant wheat seeds. One is to run the germinators at about 59° F. instead of 68° F. as previously done. Another way is to run the germinators at about 40° F. for a few days and then at 68° F. The latter method is spoken of as "chilling."

The rest period in seeds of the cereals is deeper and longer when they ripen during wet cold weather. In Germany they term barley ripening under such conditions "rain barley." The dormancy in the rain barley delayed malting unduly and various methods of treatment were worked out for shortening the rest period.

Corn that ripens during a dry fall will germinate immediately. It will even germinate on the cob if supplied moisture. Corn that ripens during wet weather has a persistent delay in germination, sometimes extending into the following seeding time. It has been found that drying the corn thoroughly just after harvest, overcomes this delay and improves the keeping quality otherwise. The artificial drying of seed corn in the autumn is becoming a general and a very paying practice in the Canadian Corn Belt. It can no doubt be adopted with great profit in the northern part of our corn belt most years and in all of the corn belt in wet ripening seasons. With our two or three billion dollar corn crop a small improvement in seed means an enormous increase in value!

Some seeds have no rest period but must grow immediately after falling or they die. This is true of the soft silver maple and other spring-seeding maples, and of cottonwoods, willows, and others. For the soft silver maple seeds it has been shown that the short life is due to drying. When these seeds fall from the tree they contain about 65 per cent. of water. When the water is reduced to 33 per cent. or less they are killed. To persist in nature they must fall where they germinate very soon and develop a root system to keep up the water supply. These seeds will retain full vitality for a year, and probably much longer, if kept moist and cold, so they neither dry nor germinate.

Many tropical seeds are short-lived and offer great difficulties in shipment in the living condition. If the loss of life in these is due to drying, this difficulty could be overcome by shipping in moist cold storage.

Why do some plants produce seeds that must germinate immediately? It is impossible to answer this question completely but the following facts about maple seeds may throw some light on it. In contrast to the spring-produced maple seeds, the autumn-produced maple seeds have dormant embryos, will withstand drying and require after-ripening in a germinator at low temperatures preparatory to germination. The fall-produced maple seeds have a winter immediately ahead of them. For safety they must rest until spring. The spring-produced maple seeds have a full growing season ahead of them. They can safely begin growth immediately and produce plants of sufficient size to withstand the winter.

See Crocker: Dormancy in Hybrid Rose Seeds (1927)